Conventional motor control systems typically require a method of measuring or sensing the current supplied to or flowing in a motor. This is because current control typically forms the "inner loop" of the motor control system, e.g., the value of the motor current is used to regulate the duty cycle of the applied voltage. In addition, current sensing is also important as a first step in determining whether a short circuit has occurred and can thus provide a means to protect the electronic power switches used in the motor control system. Determining a suitable current sensing approach is a vital step in the design of motor controls.
Typically, a servo control topology will employ a power switching matrix, such as an inverter circuit, for routing power from a d.c. voltage source to a three phase motor. One typical inverter circuit, the well known three phase bridge, consists of three parallel legs, each leg having two power switches, with a phase output being available on each leg at a point between the two power switches thereof. Commutation, i.e. control of the inverter, can be can be provided, for example, by a 120.degree. mode, 6-step, 3 phase commutation controller which causes the inverter to supply a pulse width modulated ("PWM") voltage signal to sequentially drive each phase of the motor. The pulse width modulation provides a switching cycle having a time period wherein the motor is actively driven, and a time period wherein the motor is freewheeling. The PWM duty cycle determines the average voltage applied to the motor.
The physical devices available for sensing motor load current include simple resistive shunts, current transformers, optical isolation amplifiers, and open and closed loop Hall effect transducers. One article, by Konopoka and Twitchell, entitled "Higher Voltage Servo Redesign Requires Current Sensing Re-Evaluation", Power Conversion and Intelligent Motion, February 1997, pp. 12-19, discusses the various electrical requirements that should be taken into account in selecting current and ground fault sensors for a three phase brushless servo motor controller such as described above. These requirements include isolation, linearity, zero offset, response time, bandwidth, temperature rating, hysteresis, noise immunity and insertion loss. As is typical of the prior art, the use of a resistive shunt was quickly ruled out as a current sensing option since it was assumed that a resistor would have to be serially connected to each phase of the motor and the relatively high voltage differential between each of the three phases would pose isolation problems. Instead, a closed loop Hall effect transducer was chosen as the best option because, among other criteria, it provides good isolation characteristics and the fastest response time of all the remaining current sensing devices.
Such a solution, although effective, can be relatively expensive due to the need for at least two closed loop Hall effect transducers. Other sensing options have various limitations to them such as inability to measure d.c. currents, poor accuracy, poor bandwidth, and poor response times. Despite these drawbacks, the resistive shunt, which can offer excellent performance in many of the foregoing design criteria at a very low cost, is often overlooked.
The present invention provides an approach to sensing motor current, or current flow through other types of multi-phase loads, by employing a single resistive shunt in a unique and relatively inexpensive manner which overcomes the perceived limitations of the prior art.